In some cases, it is necessary to detect a resonant frequency in a resonant space. For example, when acoustic equipment such as a speaker is installed in a hall or a gymnasium to emit a sound wave from a speaker, music or voice from the speaker is sometimes difficult to listen to because of the presence of the resonant frequency in this space (sound space in which the acoustic equipment is installed). To be specific, if the sound wave from the speaker contains a component of the resonant frequency in large amount, resonance occurs in a frequency of this component in the sound space. A resonant sound is like “won . . . ” or “fan . . . ” The resonant sound is not a sound wave to be emitted from the speaker and makes it difficult to listen to the music or the voice from the speaker.
To avoid this, the resonant frequency in the sound space is detected, and a dip filter or the like is disposed at a forward stage of the speaker in the acoustic equipment to attenuate the component of the resonant frequency. Thereby, resonance is unlikely to occur in this sound space, making it easy to listen to the music or the voice from the speaker. In order to determine a frequency characteristic of the dip filter, it is necessary to first detect the resonant frequency in the sound space.
Traditionally, an operator or a measuring person for the acoustic equipment has distinguished the sound wave from the speaker or the resonant sound depending on their senses of hearing to make judgment of the resonant frequency.